Morphological variation among geographic populations of the rare finless eels Apterichtus anguiformis and Apterichtus caecus (Anguilliformes: Ophichthidae)

Abstract Morphological analysis of recently collected specimens of the slender finless eel Apterichtus anguiformis (26 specimens) and the European finless eel Apterichtus caecus (14 specimens) in waters off the coast of Sardinia (central-western Mediterranean) confirmed their distinction from each other. Analysis also revealed a pronounced difference in mouth length in A. anguiformis from Sardinian waters when compared to the holotype and published data. To clarify their taxonomic status, morphological characteristics of the finless eels were analysed using morphometric and meristic characters. The morphometric differences between species were mainly related to the position of the anus and to cranial-related characters. Variation in meristic traits revealed differences in the number of operculum and pre-anal pores as well as the number of total vertebrae. Finally, we were able to capture specimens of these rare species using experimental trawl fishing gear, in which the entire cod-end was covered with a fine mesh net size of 8 mm; this sampling approach would be a good sampling method for further studies.